216 PROCEEDINGS OF THE VATJONAL MUSEUM. vol.38. 



equivalent to a single calyx plate. Of course many animals, as for 

 instance Boltenia, attach by a small portion of their external covering 

 which becomes pulled out into a more or less slender stalk of greater 

 or lesser length; this elongation of the external covering would carry 

 with it any calcareous structures which happened to be included in 

 it. Numerous cases of such elongation of the external body wall 

 are found in echinoderms, for instance in Caulaster, or in the Elasipoda. 



Dr. F. A. Bather believes t hat the stems of crinoids originated thus, 

 from the prolongation of the posterior part of the body of a more or 

 less irregularly plated hypothetical ancestor, the plates carried out 

 into the primitive stem becoming later regularly arranged. 1 can 

 see no reason for assuming that the stems of crinoids were derived 

 from the stems of Mastoids by any such process; they probably 

 originated independently in each. T consider the type of crinoid stem 

 composed of pentameres to represent a different sort of structure 

 from that in the recent crinoids; whereas the latter is the equiva- 

 lent of the central plate alone, the former is derived from a some- 

 what more extensive primitive base, not confined to the central 

 plate, but involving the first circlet of five plates. A stem com- 

 posed of pentameres, then, is made up of a series of repetitions of 

 the lowest circlet of plates in the crinoid calyx, and the original 

 central plate would be retained within the first of these extra cir- 

 clets laid down, that is, instead of remaining at the calyx, the central 

 plate has become fastened to the sea floor forming, as it were, a plug 

 in the end of a long tube composed of morphological repel it ions of the 

 circlet of plates surrounding it. These pentameres, as described by 

 Doctor Bather, gradually came into closer and closer contact so thai 

 eventually columnars were formed resembling those of Cdlamocrinus, 

 though morphologically entirely different. So far as I know, sections 

 of pentameres and of columnars derived through pentameres have not 

 been examined to determine the axis of crystallization. It seems 

 probable that in these cases the axis of crystallization will be found 

 to run inward direct from the periphery of the stem toward the center 

 instead of parallel to the main axis of the stem as in the other type. 



The stalk of II<>/<>]>us has been cited as an example of attachment 

 by the central plate, and of an elongation of that attachment: but 

 in reality the case is not quite so simple; in fad, ll<>h>[>nx is some- 

 thing of a combination of these two types of stem formation, for. 

 in addition to the expanded and elongated base, the basals and the 

 radials have become pulled downward so that instead of forming a 

 cup they form a tube continuous with the expanded base and join 

 with the expanded base in producing the stalk. If the stalk of 

 Holopus should become greatly elongated it is a question whether 



a RMzocrWAtS-like stem would be formed, or whether the basals 



would elongate ami. by progressively developing a series of suture-. 

 result in a stem formed of pentameres. 



